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liquid and gas. Vapor-phase HF etching at an elevated temperature has often been
used for dry etching of the sacrificial layer.
The typical Microelectronics Center of North Carolina (MCNC) (http://
www.mcnc.org/) procedure used for multiuser MEMS processes (MUMPS) runs,
which etches 2 mm thick phosphosilicate glass (PSG) layers with a minimum of 30
mm between release holes, is as follows:
1. Coat the dies with photoresist for protection in sawing, strip it by soaking in
acetone for 20 min; mild agitation is helpful.
2. Etch the PSG in fully concentrated (49%) HF at room temperature for 2.5
min, with gentle agitation.
3. Quickly transfer to DI water and rinse for 10 min.
4. Quickly transfer to isopropyl alcohol for a 5-min rinse.
5. Remove from methanol and immediately bake for 10 to 15 min at 1108C.
The importance of safely handling chemicals associated with these process
steps must be given the utmost attention. At all times the Material Safety Data
Sheets (MSDS) must be carefully read and followed.
It cannot be stressed enough that handling these materials will injure individuals
who diverge from the instructions of the MSDS. HF is one of the strongest and
most corrosive of inorganic acids. Therefore, special safety precautions are neces-
sary while using it. HF is used in a variety of industrial and research applications
including glass etching, pickling of stainless steel, removal of sand and scale from
foundry castings, and as a laboratory reagent. Exposure usually is accidental and
most likely due to inadequate use of protective measures (face shields, safety
goggles, acid gloves, and acid aprons). In the U.S., more than 1000 cases of HF
exposure are reported annually. Actual incidence rate is unknown.

13.3.3 PACKAGING
Given the huge range of MEMS applications (accelerometers, RF switches, optical
mirror arrays, etc.) contamination covers a range of issues. Consider an RF switch
when the open contacts are 300 nm apart, any particle in submicrometer range (less
than 1.0 mm) lodged in this space will obviously be a problem. Chemical contam-
ination of these surfaces can alter the electrical characteristics of the switch and
affect service life. Water vapor or other species with high surface tension can cause
stiction effects.
For reliability, the MEMS device must be isolated in a hermetic package. Often
the damage is done before packaging. Also, while hermeticity specifications are
defined in terms of leakage in and out of a ‘‘sealed’’ cavity, the issue is far more
complex. The permeation of contamination in a solution must be prevented which
occurs when contaminants diffuse through the seal over time. The outgassing must
also be limited where materials internal to the hermetic cavity (such as polymers or
epoxies) release trace quantities of gases or vapors which contaminate active
surfaces of the device.

© 2006 by Taylor & Francis Group, LLC